In a steel sheet used for a skeleton part and the like for an automobile for example, high-strength is required aiming safety against collision and reduction of fuel consumption and the like by reducing the weight of the vehicle body, and excellent formability is also required in order to be worked to a skeleton part with a complicated shape.
Therefore, provision of a high-strength steel sheet with 780 MPa class or higher tensile strength and enhanced in both elongation (total elongation: El) and stretch-flangeability (hole expansion rate: λ) is strongly desired. For example, with respect to a steel sheet of 780 MPa class tensile strength, one with 15% or more total elongation and 100% or more hole expansion rate is required, whereas with respect to a steel sheet of 980 MPa class tensile strength, one with 10% or more total elongation and 100% or more hole expansion rate is desired.
Further, one having smallest possible anisotropy (less than 1%, for example) of elongation (difference between the elongation in the rolling direction and that in the direction orthogonal to the rolling direction) is also desired.
In order to meet the needs described above, a lot of high-strength steel sheets with improved balance of elongation and stretch-flangeability have been proposed based on a variety of ways of thinking on structure control. However, the current situation is that the one in which both of elongation and stretch-flangeability are compatibly secured so as to satisfy the desired level described above has not been successfully completed yet.
For example, in the Patent Document 1, a high-strength cold-rolled steel sheet containing at least one kind of Mn, Cr and Mo by 1.6-2.5 mass % in total and composed essentially of a single phase structure of martensite is disclosed. However, in the high-strength cold-rolled steel sheet disclosed in the Patent Document 1, although 100% or more of the hole expansion rate (stretch-flangeability) is secured, the elongation does not reach 10% (refer to an example of the invention in Table 6 of the document).
Also, in the Patent Document 2, a high-strength steel sheet composed of a two phase structure including 65-85% in terms of area fraction of ferrite with the balance of tempered martensite is disclosed.
Further, in the Patent Document 3, a high-strength steel sheet composed of a two phase structure in which both of the average grain size of ferrite and martensite are 2 μm or less and the volume ratio of martensite is 20% or more and less than 60% is disclosed.
In both of the high-strength steel sheet disclosed in the Patent Documents 2 and 3, 10% or more elongation is secured however the hole expansion rate (stretch-flangeability) does not reach 100% (refer to the example of the invention of Table 2 of the Patent Document 2 and the example of Table 2 of the Patent Document 3).
Also, none of the Patent Documents 1 to 3 mentions on anisotropy of elongation.    [Patent Document 1] Japanese Published Unexamined Patent Application No. 2002-161336    [Patent Document 2] Japanese Published Unexamined Patent Application No. 2004-256872    [Patent Document 3] Japanese Published Unexamined Patent Application No. 2004-232022